1. Field of the Invention
The present invention relates generally to reproduction of information stored on a magnetic recording medium, and more particularly to magnetic storage apparatus in which information is reproduced by a magnetoresistive element in a magnetoresistive head.
2. Description of the Related Art
Magnetoresistive elements are being adopted increasingly as the information reproducing element in magnetic tape and magnetic disk storage apparatus. The magnetoresistive element undergoes a resistivity change in response to a stray field from the recording medium, a phenomenon known as the magnetoresistance effect. The reproducing sensitivity of the magnetoresistive element is higher than that of the conventional inductive element, and is substantially independent of tape velocity and disk medium rotational speed. These characteristics make the magnetoresistive element quite effective in developing smaller magnetic tape drives, improving the recording density of magnetic tape media, enhancing the capacity of magnetic disk storage apparatus, and in making smaller-diameter magnetic disk media. Thus, magnetic heads equipped with the magnetoresistive elements are being used more and more.
FIG. 1 schematically illustrates a common configuration of a magnetic recording medium (magnetic tape or magnetic disk) and a magnetoresistive element in a magnetoresistive head. Between terminals 2, the magnetoresistive element 1 forms a single magnetic domain, and a bias field is applied to magnetize the magnetoresistive element 1 in a predetermined direction in the absence of a stray field from the recording medium. This region between the terminals 2 is the magnetic sensing part, where the magnetization direction tends to rotate from the normally-biased direction when the stray field 6 from the recording medium 4 is superposed on the bias field. The resistivity of the magnetoresistive element 1 changes in proportion to the rotation angle.
Thus, this region is effective in reproducing information, and the width of this region is the reproducing track width of the magnetic head. Shielding films 3 at both sides of the magnetoresistive element 1 prevent interference between stray fields 6 corresponding to plural information items, which stray fields distinguish each item of information even when the information is recorded at a high density on the recording medium 4.
The resistivity change in the magnetoresistive element 1 in response to the stray field 6 from the recording medium 4 can be converted into an electrical signal by detecting a voltage drop between the two terminals 2 when the sense current 5 supplied to the magnetoresistive element 1 is a constant current, or by detecting a change in the sense current 5 when a constant voltage is applied to the magnetoresistive element 1.
An example of a circuit configuration that may be used as a reproducing circuit for the magnetoresistive element 1 is shown in FIG. 2. In this configuration, the voltage applied to the terminals 2 is controlled by feedback through a low-pass filter 8 so that the sense current, the value of which is defined by an external resistor element 7, is supplied to the magnetoresistive element 1. Accordingly, the sense current 5 supplied to the magnetoresistive element 1 has a constant value that is not dependent upon the resistance of the magnetoresistive element 1. In this configuration, a change in the resistance of the magnetoresistive element 1 caused by the stray field 6 causes a change in the sense current 5 corresponding to the change of resistance, since a constant voltage is applied to the magnetoresistive element 1. This change in the sense current 5 is separated out as a reproduced signal by being converted to a voltage change through a load resistor 9.
A reproducing channel (of a magnetic disk storage apparatus, for example) which utilizes magnetoresistive elements as reproducing elements has generally a composition as shown in the block diagram of FIG. 3. The sense current defining resistor element 7 is connected to a read/write amplifier 10. The reproducing signal is obtained according to the principles described above, by supplying the sense current 5 defined by the resistor element 7 to the magnetoresistive element 1. The other illustrated portions of the reproducing channel are similar to those of the conventional reproducing channel that utilizes inductive elements.
As mentioned, in the conventional technology, the resistivity change in the magnetoresistive element is detected as a change in the voltage drop in a constant-current system, or as a change in the current through the element in a constant-voltage system. Accordingly, the reproducing signal amplitude varies greatly for different magnetoresistive elements if the resistances of the magnetoresistive elements vary due to differences in the sizes of the magnetoresistive elements.
For example, Japanese unexamined patent publication number 5-325110 discloses a magnetic disk storage apparatus in which the sense current is controlled so as to maximize the reproducing level of the magnetoresistive head. Each magnetoresistive head has its own sense current to give the maximum reproducing output, as shown in FIGS. 4(a) and 4(b). However, the sense current cannot be made excessively large because a larger sense current reduces the lifetime of the magnetoresistive element. Accordingly, a practical upper limit of the sense current should be established, below which the magnetoresistive element should be operated.
A practical upper limit of the sense current can be determined, for example, by a lifetime test of many similar elements (e.g., 10-20 elements). The temperature of the element under test can be calculated from its physical dimensions (especially the height) and the current density. Then, the upper limit can be set based on the product specifications of the apparatus in which the element will be used. For example, in a magnetic disk storage apparatus employing a hard-disk drive (HDD), the predicted lifetime of the HDD is a suitable benchmark for the lifetime of the magnetoresistive element. The sense current upper limit is set accordingly.
In the following explanation, dimensions of the magnetoresistive element are defined as shown in FIG. 5. The lower portion of the, figure is the recording medium side, and the depth direction indicates the recording track width direction. As the element height (h) is determined by the machining process, it is apt to vary depending on the processing accuracy. The width (w) and thickness (t) of the element are usually determined by the thin-film process. For this reason, the variance of the element height (h) is larger than that of the width (w) or thickness (t) of the element, which tends to affect the element performance.
As the element height is reduced, the element resistance increases and the reproducing signal amplitude appearing at the element terminals increases, for a constant current. With a constant current, the current density is thus increased, and migration or destruction due to heat generation is apt to occur, deteriorating the lifetime of reliability of the device.
A graphical representation of these relationships appears in FIG. 6. FIG. 6 illustrates the reproduced output as a function of the resistance of the magnetoresistive element, which varies as a function of the element height, and the current density in the magnetoresistive element as a function of the magnetoresistive element resistance. The resistance between the terminals of the head consists of the resistance in the magnetoresistive element itself and the resistance in the terminals and wiring.
When the sense current is kept constant, the change in resistance may cause a change in the current density. For high performance, a large sense current is desired to obtain a large reproducing output. The sense current, however, must not exceed the upper limit of current density, illustrated in the lower portion of FIG. 6, to avoid reducing the lifetime of the magnetoresistive element. Therefore, the reproducing output varies in the range shown in the upper portion of FIG. 6, and for a large element height, the reproducing signal output is low because the resistance of the element is low.
Thus, it has been considered that an element having a large element height has poor performance, because its reproduced output is low due to its low resistance. However, since the current density in an element having a large element height is small, the reproduced output can be increased without deteriorating the reliability of the magnetoresistive element, by increasing the sense current in the range below the upper limit of the current density. Thus, the present inventors have discovered that the conventional technology has not fully utilized the performance of,an element having a large element height. A similar analysis can be made with respect to an element having a large element thickness.
Therefore, the present invention provides a magnetic storage apparatus which fully utilizes the performance of the magnetoresistive element, by compensating for an apparent lack of performance due to variation in the element height or element thickness.
The invention accomplishes its objectives by monitoring the voltage output between the terminals of the magnetoresistive element, and by changing the sense current supplied to the magnetoresistive element so that the voltage between the terminals is at least approximately equal to a predetermined value. The invention can be applied to various magnetic storage apparatus, including magnetic tape and magnetic disk storage apparatus.
The invention also accomplishes its objectives by changing the sense current supply to the magnetoresistive element so that the amplitude of a prerecorded reference amplitude signal reproduced from tracking information on the recording medium is at least approximately equal to a predetermined value.